Lore

Found in the Forbidden Woods, the Shadow of Yharnam consists of three humanoid creatures shrouded in black robes. As the fight progresses they are revealed to have been infected with the parasitic snakes that infest the woods and undergo gruesome mutations. They share a symbiotic link with the vipers, either transforming their arms to use them as whips or summoning gigantic serpents to devour victims.

Theories

Title of theory.

Sources

Guide Description
The Shadows of Yharnam are three faceless creatures that appear to be human but you will soon find out that they may be more. Masked in shadows and black robes, they are devoted to the Queen they serve, Mother of the Child of Blood. In their bloodless hands, they carry a weapon they once mastered. You will be up against a blood coated Chikage, a deadly blunt mace and a lethal pyromancer. Servants of the blood, they are human no more. Step close to the shadows to start the frenzy…

Madaras Whistle
Whistle of the Madaras twins, denizens of the Forbidden Woods.
The twins grew up alongside a poisonous snake, and
developed a silent, inhuman kinship.
The poisonous snake grew uncontrollably, raised on a
healthy diet of beast entrails. Even after their deaths, it is
said to respond to the call of the twins' whistle from within
the Nightmare.

Butcher Mask
Mask of the Madaras twins, denizens of the Forbidden
Woods, likely belonging to the older of the two.
The twins grew up in silent kinship with a poisonous snake.
Eventually they learned human ways, and became hunters.
When they discovered vermin even in their beloved snake,
the younger brother is said to have murdered the older.

Butcher Garb
Garb of the Madaras twins, denizens of the Forbidden
Woods, likely belonging to the older of the two.
Both the twins became hunters, and brought back and
dissected their beast prey, in order to support the villagers in
their forbidden research.

Graveguard Mask
Mask of Dores, graveguard of the Forbidden Woods.
His pale countenance mimics the labyrinth Watchers.
Willem kept two loyal servants back at Byrgenwerth.
When they were sent into the labyrinth, they encountered
the eldritch Truth, and went mad. One became the
password gatekeeper, while Dores became a graveguard
of the forest.
Both remained loyal, even in madness.

Blood Rapture
A Caryll rune that transcribes inhuman sounds.
"Blood Rapture" is the raw euphoria of the warmth of blood.
Restores HP with visceral attacks, one of the darker hunter techniques.
This rune resonates with servants of the Queen, carrier of
the Child of Blood, who yearn for their Queen's blood with
little hope of requitement.
For them, they find solace in "Blood Rapture", that serves as
a surrogate for their desires.